In existing IP (Internet Protocol) TV broadcast networks, TV channels from one or more service providers are transmitted over the network to various access nodes located in residential or commercial neighborhoods and which provide a connection to each individual subscriber premises to providing selected TV channels and other services to subscriber equipment.
Typically, the transmission media between the access node and the subscriber premises has limited bandwidth and may, for example, comprise a wire line such as a twisted copper pair. Each access node typically receives all of the channels to which customers connected to the access node subscribe. In this case, only one or a few channels may be transmitted from the access node to each subscriber premises at any one time. Therefore, when a subscriber wishes to change to another channel that is not currently being transmitted to the subscriber equipment by the access node, the channel change is performed by the access node, rather than locally at the subscriber equipment.
To reduce the bandwidth required to transport video data, the data is typically compressed prior to transmission by a video compression algorithm such as the MPEG2 standard of the International Telecommunication Unions Moving Pictures Experts Group or the H.264 of the video coding experts group, which compress video into sequences known as groups of pictures (GOPs). Each GOP contains a portion of video data which, when displayed lasts for a fraction of a second to several seconds, and is defined by respective beginning and/or ending boundaries.
Each GOP begins with a reference frame and subsequent video data in the GOP defines successive changes to the reference frame, these changes providing the sequence of video images which are ultimately displayed. A reference frame is typically required by a decoder in order to enable the decoder to start decoding the compressed video data. Therefore, the longer the time interval between successive GOPs, the longer the delay between requesting a video channel and the channel being displayed, and this delay or latency can be quite perceptible.
In order to reduce video display latency when a channel change is effected, special devices may be deployed which are dedicated to providing a portion of a previously transmitted video stream, including a reference frame to each subscriber's decoder whenever a subscriber requests a channel change. The data is transmitted to each subscriber as a unicast transmission and enables the decoder to begin decoding the video data and displaying the video image in a shorter time than would otherwise be the case if the access node simply connected the decoder directly to the multicast data stream, as this may necessitate a delay before the image can be displayed of up to about the time length of a GOP if the channel change request does not coincide with the transmission of a reference frame in the requested channel. After the requisite portion of unicast video data has been received by the decoder, the access node joins the decoder to the multicast data stream and the video data from the multicast stream is displayed after the unicast video data.
Typically, unicast data sources required for fast channel change are deployed some distance from the access node and a subscriber premises and therefore a significant amount of network resources may be required to carry the unicast data. Furthermore, as a unicast data device provides unicast data to many different subscribers, the network resources may be required to transport a large amount of unicast data. As channel changes may be concentrated at particular times of the day, for example at 30 minute or hourly intervals that coincide with the beginning and ending of programs, the network may be required to carry large amounts of unicast data at the same time, causing a significant burden and possibly even exceeding the network capacity.